Research for multiplexing image information with other image-related information is being conducted extensively. There is continuing standardization of a technique referred to as an electronic-watermark technique through which image information representing a photograph or picture, etc., is multiplexed with additional information, such as the name of a copyright holder and an indication of whether or not the image is allowed to be used, in such a manner that the additional information is difficult to distinguish visually, and the multiplexed images are distributed over a network such as the Internet. Such a watermark often is used primarily for the purpose of copyright protection.
Another field of application is the prevention of counterfeiting of banknotes, stamps and securities necessitated by improvements in the image quality afforded by image input/output devices such as copiers, scanners and printers. For example, a special mark or watermark is multiplexed with a banknote, stamp or security in advance. When the mark is sensed by an image input/output device, it is assumed that the image information is that of a banknote, stamp or security. In response, printing is halted, a warning is issued or the entire surface of the printout is intentionally covered with blank ink or the like to thereby output a degraded image that makes it impossible to use a printed copy unlawfully.
An example of embedding of a watermark will be described with reference to FIG. 5. This illustrates an example of an electronic watermarking technique through which image information is combined with a high-frequency region, etc., rather than with a region in actual space, after being converted to a frequency region using a Fourier transform.
As shown in FIG. 5, image information is first converted to a frequency region by orthogonal transform processing 501. Examples of orthogonal transforms are a Fourier transform, direct cosine transform (DCT) and wavelet transform. Next, an adder 402 adds additional information to a specific high frequency that is difficult to distinguish visually. Often the addition is to a high-frequency region because the characteristic of human vision is such that the higher the frequency region, the lower the degree of sensitivity. The signal resulting from the addition operation is returned to a region in actual space by inverse orthogonal transform processing 503. Image information in which the watermark has been embedded is thus obtained. In a case where the watermark is used in a banknote, stamp or security, a transition is made to print processing 504, at which the banknote, stamp or security having the embedded mark, which is difficult to sense visually, is completed.
FIG. 6 illustrates a procedure through which a mark is detected from the paper of the printout. As shown in FIG. 6, printed matter is read by an image reader 601 such as a scanner to input information representing the printed matter. Since the input information represents a gray-scale image obtained by print screening processing, the information is subjected to reconstruction processing 602, which is reverse screening. In general, restoration processing uses an LPF (low-pass filter). At this time the dots constituting the printed matter and scanner aliasing distortion (moiré) caused by sampling must be eliminated. Next, an orthogonal transform 603 is applied to the reconstructed information and then the embedded additional information is detected from the data of the specific frequency components by detection processing 604.
The orthogonal transformation processing and detection processing can be executed within an image reader such as a scanner, within an image output device such as a printer, within the device drivers of these peripherals, within the operating system of a host computer or within application software.
The foregoing is an example of watermarking in which a mark is embedded in a frequency region. However, there is also a method through which a mark is embedded in a region of actual space rather than in a frequency region.
Further, the specification of Japanese Patent Application Laid-Open (KOKAI) No. 7-36325 is an example of a technique for embedding a visible mark. This application proposes means for adding a mark, which is composed of a plurality of concentric circles having different diameters, to a document and detecting the mark with a high degree of precision.
The techniques mentioned above, however, have a number of problems.
Specifically, with the above-described method of detecting a watermark, processing such as orthogonal transformation requires a great deal of image memory and processing time. The same holds true with the method of embedding a mark using a region in actual space, in which differences in tonality over a wide area must be evaluated.
In the detection of a mark such as visible concentric circles or the detection of a banknote, stamp or security, matching with a pre-registered pattern is evaluated. As with the watermark, a great deal of image memory and processing time are required.
A major factor in these methods is that the purpose is to detect whether or not an embedded mark or watermark exists. That is, since these methods are premised on the fact that a mark or watermark has already been embedded in all image information, not that much processing time is required if only the type of mark is to be identified.
However, in the detection of a banknote, stamp or security or the like, information representing a large number of items of information to undergo detection processing does not contain an embedded mark or watermark. In other words, an enormous amount of time is needed to prove reliably that image information that is entirely free of an embedded mark or watermark has no embedded watermark. Further, in order to prove reliably that no mark or watermark has been embedded, it is necessary to execute detection processing a plurality of times and judge the results while changing the detection conditions.
Nevertheless, devices such as copiers solve the aforementioned problem by using a large memory and implementing detection processing by hardware.
However, when one considers an arrangement in which the above-described detection processing is executed within the printer driver of an ink-jet printer or laser printer, it is found that execution of this detection processing is not realistic owing to the limited memory available and software processing speed that is slightly slower than that of processing by hardware. If processing time of a printer driver is too long, the print processing speed of the printer engine will exceed the speed of the printer driver, resulting in a major decline in performance. For example, the printer engine may cease operating.